1. Field of the Invention
Embodiments of the present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display (hereinafter, it is also called an “LCD”) in which the viewing angle can be controlled.
2. Description of the Related Art
In general, a liquid crystal display device displays image by controlling optical transmittance of liquid crystal materials. An LCD includes liquid crystal cells arrayed in matrix, and an upper polarizing plate and a lower polarizing plate attached at both outer sides of the liquid crystal cell array in which the transmission axis of the two polarizing plates are disposed in perpendicular or in parallel to each other. A liquid crystal cell includes a liquid crystal material having a dielectric anisotropy and optical anisotropy characteristics. Further, the liquid crystal cell includes a pixel electrode and a common electrode for generating an electric field to drive the liquid crystal material. The pixel electrode is connected to a switching device, such as a thin film transistor (or “TFT”). The pixel and common electrodes can be disposed vertically or horizontally so that they can generate an electric field vertically or horizontally across the liquid crystal material. To generate a vertical electric field, the pixel and common electrodes are formed on different substrates that are facing each other with the liquid crystal material between them. To generate a horizontal electric field, the pixel and common electrodes are formed on the same substrate.
The TN (Twisted Nematic) mode, which has more merits in cost and manufacturing process, is typically used in the LCD device. The FIGS. 1A and 1B illustrate the structure of the TN mode LCD according to the related art. Referring to FIGS. 1A and 1B, the TN mode LCD 10 has an upper substrate having a common electrode 15 and an upper polarizing plate 17, a lower substrate having a pixel electrode 13 and a lower polarizing plate 11, and liquid crystal molecules 19 disposed in the cell gap between the upper substrate and the lower substrate. At an initial aligned state, the liquid crystal molecules 19 of the TN mode are stacked and arrayed with a continuous twist of 90° from the lower substrate (having the pixel electrode 13) to the upper substrate (having the common electrode 15). The upper polarizing plate 17 and the lower polarizing plate 11 are disposed such that their light transmission axes (‘a’ and ‘b’) are perpendicular each other.
The lower polarizing plate 11 plays the role of polarizing the incident light linearly. The liquid crystal molecules 19 change in their aligned or arrayed condition in response to the electric field generated between the common electrode 15 and the pixel electrode 13. As the aligned state of the liquid crystal molecules 19 is controlled, the linearly polarized light through the lower polarizing plate 11 can be controlled. The upper polarizing plate 17 plays role of transmitting the light linearly polarized in the same direction as the transmission axis of the upper polarizing plate 17.
As shown in FIG. 1A, when an electric field is not generated between the pixel electrode 13 and the common electrode 15, the liquid crystal molecules 19 maintain their initially aligned state. At this time, as the incident light 1 enters into LCD device 10 through the lower polarizing plate 11, the light components parallel to the transmission axis (a) of the lower polarizing plate 11 are passing through the lower polarizing plate 11 (linear polarization). And then, when the light passes through twisted liquid crystal materials 19, the phase is retarded so that the light linear polarization axis is rotated 90° so that it is parallel to the transmission axis (b) of the upper polarizing plate 15. Therefore, the light passing through the liquid crystal material 19 can pass through the upper polarizing plate 19 and light is presented on the screen of the LCD panel.
As shown in FIG. 1B, when a vertical electric field is generated between the pixel electrode 13 and the common electrode 15, the twisted structure of the liquid crystal materials 19 is broken by the dipole moment of the liquid crystal materials due to the applied electric field. The liquid crystal materials become aligned vertically in parallel with the electric field. At this time, an incident light 1, which is the light components parallel to the light axis (a) of the lower polarizing plate 11, is entering into the LCD. When the light components encounter the vertically aligned liquid crystal molecules 19, the polarizing state of the light components is maintained. Therefore, the light component encountering the liquid crystal molecules 19 remain polarized perpendicular to the light transmission axis (b) of the upper polarizing plate 17. As a result, light can not passes the upper polarizing plate 17 and black image is presented on the screen of LCD panel.
In the TN mode, the initial aligned state of the liquid crystal molecules 19 is that the lowest layer of the liquid crystal molecules 19 is parallel with the Y-axis of the plane of the LCD panel (when the LCD plane is corresponding to the X-Y plane of the rectangular coordinate system shown in FIGS. 1A and 1B), and the liquid crystal molecules in the next subsequent layer are twisted a little, the next layer are twisted a little more and so on with the rest of the layers of liquid crystal molecules. Finally, the liquid crystal molecules 19 in the upper most layers are parallel with the X-axis such that it is twisted 90° with respect to the lowest layer of liquid crystal molecules. In this case, the LCD device 10 has a gray inversion problem in which the contrast in the up, down, left and right viewing angles is inversed. To solve this problem in a typical LCD panel using TN mode liquid crystal material, the lowest liquid crystal molecules are aligned at a −45° direction from respect to the Y-axis of the LCD panel 10. From the lowest layer of liquid crystal molecules, the aligned directions of the liquid crystal molecules in the layers of liquid crystal molecules are twisted continuously to 90° as the layers go upward so that the aligned direction of the liquid crystal molecules in the upper most layer is aligned at +45° with respect to the Y-axis of the LCD panel 10. This is determined by considering that the most users of LCD panel are seeing the panel from the 6 o'clock direction of the panel. In accordance with the alignment direction of the TN mode liquid crystal molecules 19, the transmission axis of the upper polarizing plate 17 should be disposed at +45° with respect to the Y-axis of the LCD panel 10. Here, the Y-axis is generally the lateral direction (long axis) of the LCD panel 10.
Most LCD devices including TN mode LCD panel 10 are developed to have wide viewing angle so users can see the normal display information of the screen from any viewing angle. However, some fields, such as banking or personal information conveyance, the viewing angle should be narrow in range or controlled to be narrow in range. Therefore, hybrid types viewing devices in which user can select the viewing angle according to the using environment or purpose have been developed.
Currently, a viewing angle controllable LCD device includes an LCD panel (also called as ‘Screen Panel’) and a viewing angle controllable LCD (also called as ‘View Angle Panel’) attached thereon. The view angle panel controls the viewing angle of the display information from the screen panel. For example, when the view angle panel is not activated by an electric field, the viewing angle controllable LCD operates in the wide viewing angle mode so that user can see normal screen data in any direction of front of the LCD device. However, when an electric field is applied to the view angle panel, the viewing angle controllable LCD is operates to narrow the viewing angle mode of the LCD panel so that only the user right front of the LCD panel can see screen data.
The viewing angle controllable LCD device should control the viewing angle in bilateral symmetry for the most effective viewing control quality. However, as explained in above, a TN mode LCD panel 10 typically has polarizing plates with transmission axis that are at +45° and −45° to the Y-axis, respectively. Therefore, the light past through from the TN mode liquid crystal molecules is polarized to at +45° direction to the Y-axis. As a result, the view angle panel can not control the viewing angle of the polarized light by the TN mode LCD panel with bilateral symmetry. In other words, for an initial state of a TN mode LCD panel in which the transmission axis of the polarizing plates are aligned at 0° and 90° with respect to the Y-axis, it might be possible to make bilateral symmetry viewing angle controllable LCD using TN mode LCD. If it was, the gray inversion problem would not be solved. For this reason, a viewing angle controllable LCD device for a TN mode LCD, which does not have the gray inversion problem, has not been developed.